Optical switches, which can directly manipulate optical signals, are becoming increasingly important for optical networking. Accordingly, several techniques for switching optical signals have been developed. FIG. 1 shows a plan view of an optical switch 100 that uses some of the optical switching techniques described in U.S. Pat. No. 5,699,462 to Fouquet et al., entitled “Total Internal Reflection Optical Switches Employing Thermal Activation.” As illustrated in FIG. 1 and in the cross-sectional views of FIGS. 2A and 2B, optical switch 100 includes a planar lightwave circuit 110, a semiconductor substrate 120, a base plate 130, and a reservoir 140.
Planar lightwave circuit 110 is a plate of an optical material such as quartz containing crossing waveguide segments 112 and 114 and cavities 116 at the intersections of waveguide segments 112 and 114. Optical signals are generally input to optical switch 100 on one set of waveguide segments 112 or 114, and cavities 116 act as switching sites for the optical signals. In particular, a cavity 116 when filled with a liquid 142 having a refractive index matching the refractive index of the waveguide segments 112 and 114 transmits an optical signal from an input waveguide segment 112 or 114 into the next waveguide segment 112 or 114 along the same path. FIG. 2A shows a cavity 116 filled with liquid 142 from reservoir 140.
A cavity 116 becomes reflective for switching or redirection of an optical signal when the cavity contains a bubble. More specifically, total internal reflection at an interface 115 between an input waveguide 112 or 114 and a bubble 146 (as shown in FIG. 2B) switches an optical signal into a crossing waveguide segment 114 or 112. Selectively activating or making reflective one of the cavities 116 along the initial path of an optical signal can switch the optical signal onto any of the crossing waveguide segments 114 or 112. If none of the cavities 116 along the path of an optical signal are reflective, the optical signal passes straight through optical switch 100.
Semiconductor substrate 120 contains electronic circuitry that includes heating elements 122 positioned in cavities 116. Selective activation of a heating element 122 vaporizes liquid in the corresponding cavity 116 and makes reflective the switching site corresponding to the cavity 116 containing the activated heating element 122. The activated heating element 122 then continues heating to keep the bubble stable and the switching site reflective. If the heating element 122 is turned off, bubble 146 and surrounding liquid 142 cool, causing bubble 146 to collapse and the cavity 116 to refill with liquid 142.
Base plate 130 acts as a heat sink for semiconductor chip 120 but also includes an inlet 136 connected to reservoir 140. Inlet 136 and a hole 126 through semiconductor substrate 120 allow liquid 142 to flow between reservoir 140 and a gap 118 underlying cavities 116. When a bubble 146 forms or collapses to activate or deactivate a switching site, liquid 142 flows to or from reservoir 140 via gap 118, hole 126, and inlet 136.
Reservoir 140 is partially filled with liquid 142 and partially filled with a gas 144, typically vapor from liquid 142. The pressure of gas 144 controls the fluid pressure of liquid 142 and therefore controls the difficulty of forming bubbles in cavities 116. U.S. Pat. No. 6,188,815 issued Feb. 13, 2001 to Schiaffino et al., entitled “Optical Switching Device and Method Utilizing Fluid Pressure Control to Improve Switching Characteristics,” describes how a pressure controlling mechanism in reservoir 140 can elevate the pressure of liquid 142 to avoid inadvertent formation of bubbles that might cause improper switching in switch 100.
Optical switches similar to switch 100 have proven effective for switching optical signals. However, improvements are sought in several areas. Energy consumption, for example, in switch 100 can be significant when several switching sites are simultaneously reflective. To keep a switching site reflective, the corresponding heating elements 122 must locally maintain a temperature high enough to prevent collapse of the bubble 146 in the overlying cavity 116. This constant drain of energy continues even when the routing of optical signals through optical switch 100 remains constant. The energy consumption also generates heat that can be difficult to dissipate, particularly in compact optical switches having a high density of heating elements 122. The heating, being localized to small areas, can lead to damage and failure of electronic circuitry. For a practical device having a commercially useful lifetime, the amount of heating must be limited which in turn limits the types of liquid that an optical switch can use. Specifically, some liquids require too much heating to create and maintain a bubble and cause conventional heating elements to burn out quickly.
Another concern for optical switch 100 is condensation and distillation that can occur in cavities 116 containing bubbles 146. Each bubble 146 is kept at a locally elevated temperature to maintain the vapor pressure inside bubble 146 and thereby prevent bubble 146 from collapsing. The heated vapor in the bubble 146 can condense onto the cooler walls of the cavity 116. Condensation at interface 115 between a cavity 116 and an input waveguide segment 112 or 114 can cause off-angle reflection, resulting in signal loss when less of the optical signal reflects into the desired output waveguide segment 114 or 112 and resulting in noise or cross-talk if part of the optical signal enters other waveguide segments.
Evaporation and condensation can also cause local fractional distillation when liquid 142 contains two or more separable compounds. The fractional distillation can locally change the composition and therefore the refractive index of liquid 142. Having matching refractive indices for liquid 142 and waveguide segments 112 and 114 is critical to avoiding intolerable levels of reflection at switching sites intended to be transparent. The distillation problem limits the suitable choices for liquid 142 to liquids that resist distillation that changes the liquid's refractive index.
In view of the limitations in existing optical switches, there is a need for structures and operating methods that expand the choices for suitable liquids in optical switches and reduce power consumption and heat generation in optical switches.